Many gases are typically stored as liquids, or dissolved in a solvent therefor. For example, nitrogen is normally stored as a liquid, whereas acetylene is normally stored by being dissolved in a solvent such as acetone. Both liquid gas and gas solutions are stored in a heat hardened and dried monolithic calcium silicate filler mass having very fine pores so as to provide a porosity of at least about 85% and, more desirably, at least about 88%. This means that 85% to 88% of the volume of calcium silicate mass comprises pores. This monolithic filler mass is formed in a metal shell and the pores of the monolithic filler mass are filled with either a liquid gas or a gas solution for storage and/or transporting. Typically, the porous monolithic filler mass is formed from an aqueous slurry of silica and quicklime (CaO), in the proportion of ten parts of lime (CaO) and 10 to 15 parts of silica (SiO.sub.2). The aqueous slurry is poured into the metal shell and autoclaved at high temperatures and saturated steam pressure to form a hardened monolithic calcium silicate filler mass. The hardened calcium silicate filler means is then baked in an oven to drive the water from the hardened calcium silicate thereby obtaining a certain porosity and form the heat hardened and dried monolithic calcium silicate filler mass.
Two of the most important properties that a filler mass for storing liquid gases or gases dissolved in a solvent must possess are the porosity of the filler mass and the compressive strength of the mass. The porosity of the filler mass is important because the amount of porosity is directly related to the amount of gas which may be stored. A small increase in the porosity of a filler mass greatly increases the amount of gas which may be stored therein. In addition to the amount of porosity, the type of pores creating the porosity is also critical. In the case of an acetylene gas dissolved in a solvent such as acetone, the gas solution is stored in the pores of the monolithic heat hardened calcium silicate filler mass which, as noted, is located in a closed metal shell which is normally in the shape of cylinder. The pores in the filler mass must be disposed substantially, uniformly throughout the filler mass and are generally very fine, having a size of between 0.05 microns to about 25 microns.
In addition to porosity, another very important requirement of the hardened and dried monolithic filler mass is that it have a high compressive and tensile strength so that the storage vessel may withstand the rough handling it receives. For example, gas storage vessels are often dropped, which if the filler mass did not have a high compressive or tensile strength would cause structural failure or destruction of the filler mass. Such structural failure in the case of explosive gases, can be very dangerous. For example, such structural failures can result in large void spaces in the filler mass which could cause an explosion. In addition, such structural failure can clog the various fluid paths in the storage vessel with a buildup of pressure which can also cause an explosion.
Therefore, persons in the art have made numerous attempts to increase the structural strength and at the same time maintain or increase the porosity of the calcium silicate monolithic filler mass. For example, the prior art has added asbestos fibers to the calcium silicate filler mass in order to increase the structural strength of the filler mass and at the same time, maintain the other desirable and necessary properties of the calcium filler mass. See for example U.S. Pat. No. 2,883,040. In general, such filler masses which have anywhere from 10 weight percent to 20 weight percent asbestos fibers uniformly dispersed throughout the calcium silicate filler mass have performed satisfactorily. However, though the porosity of such silicate filler masses has been satisfactory, the compressive strength is not as high as the art would like and, in addition, in recent times it has been shown that asbestos may be dangerous to a person's health. Therefore, experiments have been conducted in an attempt to utilize fibers other than asbestos. For the most part, such experiments were unsuccessful because producing an acceptable silicate monolithic filler mass is a "black art", and it is impossible to predict whether a given fiber can produce a calcium filler mass having all the properties necessary to have a safe and effective storage vessel for liquid gases or gas solutions. Thus, it is a very difficult and time consuming task to evaluate fibers to determine if the inclusion of such fibers in a calcium silicate filler mass would produce an adequate filler mass.
After much time and effort, it was determined that alkali resistant glass fibers can be uniformly dispersed throughout a monolithic calcium silicate filler mass to produce an acceptable storage vessel for storing liquid gases and gases in solutions (see for example U.S. Pat. No. 4,349,463). However, the filler mass disclosed in this patent, even though being satisfactory, still could be improved in its porosity and structural strength. In this regard, it is noted, that in general, the amount of porosity, (i.e., the percent of the volume of the calcium silicate mass which is composed of pores) is generally determined by the amount of water used in preparation of the slurry and then driven off during the autoclaving step and baking step. However, to some extent, the porosity is also determined by the fibers utilized in increasing the structural strength of the filler mass.
As noted before, void spaces within the storage vessel must not be present in order to avoid the danger of explosion due to acetylene gas decomposition in these void spaces because of fire and/or flashback. It is therefore very important that the closed metal shell, (e.g., a metal cylinder) be substantially completely filled with the filler mass. In general, it can be said that the overall clearance between the metal shell and monolithic calcium filler mass must not exceed 1/2 of 1% of the respective diameter or length, but in no case to exceed 1/8 inch measured diametrically and longitudinally. The art has generally recognized that for safety considerations there should not be more than 1/8 of an inch between the calcium silicate filler mass and the metal shell. In normal practice, in producing storage vessels for liquid gases and gases in solution, an aqueous slurry of silica and quicklime (calcium oxide) is placed in the metal shell and then autoclaved and dried to form the monlithic filler mass within the shell. Since, as noted above, clearance between the shell and filler mass should not be greater than 1/8 of an inch, it is very important that during the hardening and drying, the filler mass should not appreciably shrink. Thus, any fibers utilized to increase the structural strongth of the monolithic filler mass should cause very littly shrinkage of the filler mass during autoclaving and heating. However, during autoclaving and heating there can be some minimal shrinkage. The reason for this is that it is desirable to have some clearance between the filler mass and metal shell in order to enhance gas discharge characteristics of the storage vessel. However, in general, it is thought that the less shrinkage the better.
In addition to the foregoing requirements of a hardened monolithic calcium silicate filler mass, the filler mass should also have at least 50 weight percent of crystalline phase (based on the weight of the calcium solicate) and preferably at least 65 or 75 weight percent crystalline phase. This is important in order to have good compressive strengths and also to reduce shrinkage at the high temperatures utilized in producing the filler mass. Therefore, the use of a fiber for increasing the compressive strength should not adversely affect the formation of a crystalline phase. In this regard, it is noted that in general during the autoclaving and the baking of the filler mass, there are various crystalline phases formed. These crystalline phases are tobermorite, xonotlite and quartz. There is also formed an amorphous phase which should be minimized as much as possible.